1. Field of the Invention
This invention relates to a method for converting analog voltage to digital voltage, as well as an analog-to-digital converter. In particular, this invention relates to a method for digitally eliminating offset voltage errors etc. as well as an integrating type analog-to-digital converter particularly suitable for LSI version.
2. Description of the Prior Art
An analog-to-digital converter including a means for digitally eliminating conversion errors resulting from offset voltage is disclosed in U.S. Pat. No. 3,872,466 specification. The analog-to-digital converter has various excellent advantages. For example, it suffers no hysteresis and propagation delay of a comparator in the converter. A reference voltage necessary for conversion may be of a unipolar type and is suited to LSI version. Conversion around zero is continuous. The converter suffers no influence resulting from offset voltage of an integrator etc. However, such a convertor is susceptible to averse influence of dielectric absorption and insulative resistance of an integrating capacitor, because an unknown analog voltage and reference voltage are superposed one over the other. For the positive and negative unknown analog voltages the output waveform of the integrator in the converter is completely asymmetric with respect to the time base. Therefore, there is a cause for noncoincidence between the absolute values of a conversion value of the unknown analog voltages having an opposite polarity and equal absolute value. The zero point of the converter is continuous, but there also exists a cause for difficulty of obtaining an accurate zero point. The integration time to the unknown analog voltage varies dependent upon the magnitude of offset voltage errors and thus is not constant.